P1 B1 CTB Physiology of the Large Intestine PDF

Physiology of the Large Intestine Dr Seley Gharanei, Assistant Professor Cell and Tissue Biomedicine [email protected] MB ChB Phase I Block 1 Health, Metabolism and Homeostasis  Learning Outcomes  Outline the structure and functions of th...

Physiology of the Large Intestine Dr Seley Gharanei, Assistant Professor Cell and Tissue Biomedicine [email protected] MB ChB Phase I Block 1 Health, Metabolism and Homeostasis  Learning Outcomes  Outline the structure and functions of the large intestine  Outline the mechanisms of Na+ and water reabsorption in the large intestine  Describe the movements of the colon and the action of taeniae coli  Outline the reflexes associated with defaecation  Outline the actions of the bacterial populations of the gut Large Intestine (Colon)  1.5 m long, larger diameter (6 cm)  The caecum follows a blind ended pouch from which extends the appendix.  Ascending, transverse, descending  The terminal portion of the descending colon is S-shaped, the sigmoid colon, empties into rectum - anus  Many crypts, mucus glands and goblet cells. No villi (limited nutrient absorption)  Motility is stimulated by mastication and hormones gastrin and cholescystokinin. Inhibited by glucagon. Functions of the Large Intestine 1. To extract Na+ and water from the luminal contents. 2. To make and store faeces. 3. To move faeces towards the rectum. reabsorb water; maintain osmolarity (secretion/absorption of electrolytes (Na+, Cl-); store fecal material (indigestible matter, mucus, Q1: what happens when the dead bacteria) large intestine absorbs less water? Vander et al. Human Physiology, McGraw-Hill Q2: what happens when the large intestine absorbs too Secretions of the Large Intestine The luminal epithelium of the large intestine secretes an alkaline mucus (from Goblet cells). This serves to:  protect the epithelium from acid, abrasion, and bacterial activity.  provide an adherent medium for holding faecal matter together. Q3: What GI tract disease causes inflammation in the lining of the colon, leading to diarrhoea, blood in the stool, abdominal pain, and weight loss, among other symptoms? Fluid Balance in the GI Tract Q4: what bacterial disease is known to cause tremendous loss of fluid ?  Electrolyte Transport and Water Reabsorption in the Key Concepts: Intestine  NET flux of Na+ and water is from the lumen to the blood.  Absorption is an active process.  The driving force for water absorption is the movement of Na+.  Water follows osmotic gradients.  The Na+/K+ ATPase plays a key role.  Electrolyte transport processes and routes of water movement vary from one segment of the intestine to another.  Electrolyte Transport and Water Reabsorption epithelial cell of the jejunum in the Jejunum luminal membrane basolateral membrane Na+ exits the Na + cell via the Na+-K+ ATPase Na+ enters the cell on coupled transporters lumen blood water Water crosses the epithelium via a mainly paracellular route Image: Costanza, Physiology (Elsevier)  Movement of Water and Electrolytes Across the Intestinal Epithelium “paracellular” “transcellular” lumen blood tight junction The tight junctions between intestinal epithelial cells are “leaky”. duodenu colon m least tightest tight  Electrolyte Transport and Water Reabsorption in the Colon epithelial cell of the colon basolateral luminal membrane membrane Na+ exits the Na + cell via the Na+-K+ ATPase Na+ enters K+ enters the the cell cell via the through a Na+-K+ Na+ channel ATPase water lumen blood K+ exits the cell through a K+ channel Water crosses the epithelium via a Image: Costanza, Physiology (Elsevier) transcellular route  The Ileocaecal Valve ileocaecal sphincter valve pushes valve shut pushes valve open  separates the terminal end of the ileum from the caecum.  is a one-way valve guarded by a sphincter.  prevents back-flow of faecal contents from the colon.  controls the rate at which ileal chyme enters the colon.  is regulated by neural and hormonal mechanisms. Image: Vander et al. Human Physiology (McGraw-Hill)  Movements of the Colon The motility of the colon:  Facilitates the efficient absorption of water and salts.  permits the orderly evacuation of faeces. There are 2 types of movements in the colon:  Mixing: “Haustral Contractions”.  Propulsive: “Mass Movements”.  Haustral Contractions  The longitudinal muscle of the colon is aggregated into 3 bands semi- mush mush called the taeniae coli. semi- fluid  Combined contractions of the fluid taeniae coli and the circular solid muscle layer causes the colon to bulge into ovoid segments called haustrae. hard solid  Haustral contractions (segmented) mix the contents of the proximal colon. This facilitates Image: Berne & Levy, Principles of Physiology, (Mosby)  Segmentation  Cycles of contraction that mix contents but do not push them in any one direction.  In the GI tract, churn and fragment the bolus. Mix contents with intestinal secretions. Image: Martini et al., Human Anatomy (Prentice Hall)  Haustral Contractions  The longitudinal muscle of the colon is semi- mush mush aggregated into 3 bands called the semi- taeniae coli. fluid  Combined contractions of the taeniae fluid coli and the circular muscle layer causes solid the colon to bulge into ovoid segments called haustrae. hard solid  Haustral contractions (segmented) mix the contents of the proximal colon. This facilitates water absorption. Image: Berne & Levy, Principles of Physiology, (Mosby)  Mass Movements semi-  Propel the luminal contents mush mush semi- from the beginning of the fluid transverse colon to the sigmoid colon. fluid solid  Are a series of modified peristaltic events. hard solid  Occur 1-3 times a day. Image: Berne & Levy, Principles of Physiology (Mosby)  Peristalsis  Waves of alternating contractions and relaxations of smooth muscle layers that mix and squeeze the contents through hollow tubes.  Move a bolus along the length of the GI tract. Image: Martini et al., Human Anatomy (Prentice Hall) Recall: P1 B1 CTB Digestive system  Mass Movements semi-  Propel the luminal contents mush mush semi- from the beginning of the fluid transverse colon to the sigmoid colon. fluid solid  Are a series of modified peristaltic events. hard solid  Occur 1-3 times a day. Image: Berne & Levy, Principles of Physiology (Mosby)  1. The Gastrocolic Reflex  is mediated by gastrin and by the extrinsic autonomic nerves.  initiates mass movements in the colon.  pushes the colonic contents into the rectum, triggering the defaecation reflex. 2. The Defaecation Reflex  Rectum and Anal Canal  Are normally empty of faeces.  The anal canal is guarded by 2 sphincters: 1. internal anal sphincter 2. external anal sphincter Image: Smith & Morton, The Digestive System (Churchill Livingstone) Image: Smith & Morton, The Digestive System (Churchill Livingstone)  Rectum and Anal Canal  Are normally empty of faeces.  The anal canal is guarded by 2 sphincters: 1. internal anal sphincter 2. external anal sphincter Image: Smith & Morton, The Digestive System (Churchill Livingstone) The Defaecation Reflex a reflex response to distension of the rectal wall. mediated by mechanoreceptors. The defaecation reflex response consists of: 1. Contraction of the rectum. 2. Relaxation of the internal anal sphincter. 3. An initial contraction of the external anal sphincter. 4. Increased peristaltic activity in the sigmoid colon. 5. Relaxation of the external anal sphincter. 6. Expulsion of faeces.  Voluntary Control of Defaecation  Brain centres can override the reflex signals and thus keep the external sphincter closed.  Voluntary defaecation involves the Valsalva manoeuvre: The Valsalva Manoeuvre 1. Full inspiration followed by forced expiration against a closed glottis causes the diaphragm to move downwards. 2. The abdominal and thoracic muscles are contracted. 3. Increased pressure in the abdomen forces faecal contents into the rectum.  Composition of Faeces 3/4 water 1/4 solid matter: 30 % dead bacteria 10-20 % fat 10-20 % inorganic matter 2-3 % protein 30 % undigested material, including bile pigment and sloughed epithelial cells. Brown colour: derivatives of bilirubin (stercobilin, urobilin) Odour: products of bacterial action (include skatole, indole, mercaptans and hydrogen sulphide)  Intestinal Bacteria The large intestine harbours a large population of anaerobic bacteria, which serves several physiologically useful functions:  provides immunity against common bacteria  converts bilirubin to urobilinogens  forms secondary bile acids  degrades digestive enzymes  digests mucus Q6: Why do intestinal bacterial  synthesizes certain vitamins (e.g. vitamin K) digest certain nutrients and what by product do you get as a  metabolizes undigested polysaccharides results? All of these functions are disrupted by the administration of oral antibiotics.  Summery  The structure and functions of the large intestine  The mechanisms of Na+ and water reabsorption in the large intestine  The movements of the colon and the action of taeniae coli  The reflexes associated with defaecation  The actions of the bacterial populations of the gut  Recommended Reading Medical Sciences, Naish and Syndercombe Court, Chapter 15: The Alimentary System https://0-www-clinicalkey-com.pugwash.lib.warwick.ac.uk/student/content/ book/3-s2.0-B9780702073373000158#hl0003408 Section: Large Intestine  Answers to questions 1. : what happens when the large intestine absorbs less water? Diarrhoea Q2: what happens when the large intestine absorbs too much water? Constipation 3. Ulcerative colitis affects the large intestine, causing inflammation involves the lining of the colon, leading to diarrhoea, blood in the stool, abdominal pain, and weight loss, among other symptoms. Q4: what bacterial disease is known to cause tremendous loss of fluid ? Cholerae Q5: Why do intestinal bacterial digest certain nutrients and what by product do you get as a results? Because we lack the enzyme to digest certain those nutrients. CH4 and H2S methane and hydrogen sulphate due to digestion of carbs by the intestinal bacteria